The fundamental theory upon which spread spectrum communication relies is Shannon formula, which is:C=B log2(1+S/N)
where C is the system channel capacity (bit/s), B is the bandwidth of the system channel, N is the noise power, and S is the signal power. Shannon formula demonstrates a relationship between the capability for a system channel to transmit information errorlessly, the signal-to-noise ratio (S/N) existing in the channel and the system channel bandwidth (B) for transmitting information. This indicates a trade-off between the channel bandwidth and the signal-to-noise ratio under the condition of certain channel capacity. In communication, the anti-interference capability can be improved by either narrowing the channel bandwidth and increasing the signal power (i.e., increase the signal-to-noise ratio) or reducing the power of the transmitting signal and broadening the channel bandwidth. Spread spectrum communication is a communication system that employs the latter one.
FIG. 1 is a schematic block diagram of a spread spectrum modulation and demodulation system. A code generator 102 of a transmitting side generates spread spectrum code chips to spread the original data in a spreading unit 101. Modulation of the spread high-speed data is conducted in a modulation unit 103 in which a transmitting data adapted for the channel is generated. After going through the interference imposed by the channel 104, the transmitting data arrives at a receiver. The receiving side first demodulates the receiving data utilizing a demodulation unit 105, and then performs despreading operation by utilizing a despreading unit 106 and local spread spectrum code chips generated by a code generator 107 of the receiving side, and thus the original data is eventually recovered.
The types of spread spectrum includes direct sequence spread spectrum, frequency hopping, time hopping, etc. The first type—direct sequence spread spectrum is now a most widely used one. The direct sequence spread spectrum communication employs a spread spectrum code sequence to implement the spreading of the frequency spectrum. Generally, the spread spectrum code is implemented by a pseudo random sequence, for example, m sequence, gold sequence, etc. The spread spectrum code chips used in the third generation mobile communication system is walsh code+gold code, wherein the walsh code accomplishes orthogonal feature and the gold code applies interference upon the walsh code so as to level the noise. Although such code chips have a sound self-correlation property, its cross-correlation property is not that good. Users in different cells are subject to severe multi-access interference from the neighboring cells since it is impossible to have the spread spectrum code chips for difference cells synchronized.